It’s no news that Mars is teeming with extinct volcanoes. The biggest, Olympus Mons, towers some three times higher than Mount Everest, spreads so wide that it could cover most of France and is so prominent that its snowy summit was first spotted from Earth in the late 1800’s. Astronomers now know of literally thousands of smaller volcanoes as well, sprinkled liberally around the Red Planet, some of them as much as 3.5 billion years old.

But there must have been volcanism before that. Mars formed 4.5 billion years ago, and babyhood is a turbulent time for planets. “Early in its history,” says Joseph Michalski, of the Planetary Science Institute in Tucson, Arizona, “Mars would have been hot inside.” So by the time the volcanoes we can see today formed, the planet should already have been releasing heat from its molten interior for a billion years. “The problem,” says Michalski, “is that we haven’t had any empirical evidence for it.”

They do now. According to a new paper Michalski co-authored in the latest Nature, at least some of Mars’ earliest volcanic activity came in the form of gigantic, explosive supervolcanoes that blasted hundreds or even thousands of square miles of territory into the air with the force of a thousand Mount St. Helens eruptions—or even more.

The evidence comes from the Arabia Terra region in Mars’ northern hemisphere, where planetary scientists have long noted a peculiar type of crater dotting the landscape. “They’re just massive holes in the ground,” says Michalski, “up to 50 miles [80 km] across.” They could have come from meteor impacts, except, he says, those tend to produce other features, like a raised rim, a central peak and sprays of excavated material spreading outward from the crater. Those telltale markings and formations might have eroded over time, but, says Michalski, “when that happens the basins tend to fill in, and these are quite deep.”

It’s more likely that these craters are similar to sinkholes created by the collapse of the surface. “One way that would happen,” Michalski says, “is if there were a bunch of ice under the surface and it were removed somehow.” In fact, he says such ice-collapse features are known to exist on both Earth and Mars, but, he says, “these are orders of magnitude larger. There would be just too much ice to make it plausible.”

But such collapse features can also happen when a supervolcano erupts, emptying the subsurface of hot magma. That very purging means that the new surface that forms after an eruption isn’t firmly supported from beneath, and parts of it eventually fall into the weak spots. That scenario not only explains the size and shape of the craters; it may also explain the presence of fine-grained ash deposits found throughout Mars’s equatorial region. It appears volcanic in origin, but there’s far too much of it for any of the known volcanoes on the planet to have produced.

A supervolcano could explain everything, and the effects would have gone well beyond simply adding a new layer to the Martian landscape. Back then, Mars had a relatively thick, heat-retaining atmosphere that allowed liquid water to exist comfortably on the planet’s surface—and might have supported life too. Before all that dust settled onto the ground, it would have lingered in the atmosphere for weeks or months, shielding the planet from the Sun’s warmth and plunging Mars into a temporary deep-freeze.

That would have been similar to Earth’s year without a summer caused by an Indonesian volcano in 1257, as well another deep-freeze triggered by another Indonesian volcano in 1815—except much, much worse. Fortunately for Mars, the planet has cooled considerably in the last 3.5 billion years, so major eruptions are almost certainly a thing of the past. (Unfortunately for Mars, the the loss of nearly all the planet’s atmosphere means has left it permanently frigid and dry.)

Earth, however, remains a geologically volatile place. There’s a potential supervolcano simmering in Italy, and another underneath Yellowstone National Park. If either one erupted, the devastation would be pretty much unimaginable. How likely is that to happen? Back in 2009, geologists at the Yellowstone Volcano Observatory assured TIME that a supervolcano “does not appear imminent.” That’s some comfort, though a fair bit less than most of us—to say nothing of our descendants—might like.

Mars Like You’ve Never Seen It Before

NASA / JPL / University of Arizona

Defrosting of the Crests of Inca City

Audaciously called Inca City because of its rectangular, geometric shapes evoking vestiges of habitats, these terrains have nothing reminiscent of a city or of the Inca. Their size—several kilometers—is the best proof of that. The nature of this polygonal network, unique on Mars, remains poorly understood, but seems to be linked to volcanic dykes. These terrains are close to the South pole and undergo springtime defrosting in dark patches that become progressively larger as temperatures climb.